This invention relates to a paper feed mechanism for use in an image forming apparatus such as a copying machine, a facsimile machine, and a printer.
Description of the Background Art
Conventionally, there has been known a paper feed mechanism of a type in which a cross sectional D-shaped roller having a contact area (round portion) for pressingly coming into contact with a copy paper and a non-contact area (flat portion) is employed to feed copy paper, stacked on paper support means such as a paper cassette, one by one into an image forming apparatus along in a sheet transport direction (Japanese Unexamined Patent Publication No. HEI 5-330682). The paper feed mechanism disclosed in this publication can prevent multifeed of copy paper by a pair of separation pawls which are provided on widthwise opposite ends of a lead portion of the paper support means.
Another type of paper feed mechanism is proposed in which a first roller, similar to the above D-shaped roller and a second roller disposed below the first roller, are used to feed copy paper in the sheet transport direction by nipping the copy paper between the first and second rollers while rotating the same in the forward direction (sheet transport direction). In this mechanism, multifeed of paper is prevented by mounting a torque limiter to the second roller.
The paper feed mechanism with the torque limiter functions as follows. When copy paper is not nipped between the first and second rollers, the second roller rotates in a reverse transport direction opposite to the sheet transport direction together using the torque limiter, thereby preventing the copy paper not in a nipped state from being inadvertently fed in the sheet transport direction. On the other hand, when copy paper is in be fed to the sheet transport direction in a nipped state between the first and second rollers, the second roller is rotated in the forward direction following the rotation of the first roller. In this way, multifeed of copy paper is prevented.
The above paper feed mechanism with the torque limiter has the following problem. In order to initiate the forward rotation of the second roller accompanied by the rotation of the first roller, the first roller and second roller are required to be already in a pressing contact state for nipping copy paper therebetween. This means that feed of copy paper is initiated when the motion of the second roller is still in an unstable state immediately after it is shifted from the reverse transport rotation to the forward transport rotation. That is, the feed of copy paper becomes unsteady, resulting in a failure to transport the copy paper toward rollers arranged downstream, such as transport roller pair, and registration roller pair as timed with the feed of copy paper from the paper support means.